Polyolefins and other polymer types may be fabricated into films by either of two general film forming techniques. Molten polymer may be extrusion cast through a slot die to form a film layer, and films so formed are generally referred to as cast films. Or, molten polymer may be extruded through an annular die to form a gas tight enclosure of the extrudate which is then blown filled with air to expand the extrudate into an air supported film bubble, and films so formed are generally referred to as blown bubble films.
The technique of forming polymer resins into films by a blown bubble extrusion technique is widely practiced and presents various processing simplifications and conveniences compared to that of film formation by slot die extrusion casting techniques. However, to successfully practice film formation by a blown bubble extrusion technique, the polymer resin of which the film layer is to be formed must possess certain minimum physical/mechanical properties, chief of which is a strength at its extrusion temperature (i.e., "melt strength") sufficient to support formation of a film bubble during its blowing and expansion by air.
Heretofore certain types of polymer resins that otherwise possess physical/mechanical/chemical properties that are desirable in a film for various end uses, have presented melt strength properties that render such resins problematic for production into films by a blown bubble extrusion technique. One such type of problematic polymer resin is that of linear low density polyethylenes. A linear low density polyethylene, conventionally referred to as LLDPE, is a copolymer of ethylene with a minor quantity of an olefinic hydrocarbon comonomer, typically a acyclic C.sub.3 -C.sub.8 alpha-olefin, such that ethylene comprises at least about 80 wt. % of the polymer whereas the comonomer content comprises less than about 20 wt. % of the polymer mass. Copolymerization of ethylene with such minor quantities of acyclic olefinic hydrocarbon comonomer introduces short chain branching along the polymer backbone to yield an ethylene based polymer having a density in the range of about 0.910 to about 0.940 g/cm.sup.3, with lower densities associated to higher comonomer contents and higher densities associated to lower comonomer contents. An LLDPE thus possesses many mechanical/chemical property attributes that are similar to a highly branched low density homopolyethylene produced by high pressure free radical polymerization while also possessing certain mechanical/chemical and rheological properties like that of a high density linear homopolyethylene produced by low pressure Ziegler-Natta polymerization processes. Hence, this high ethylene content ethylene-alpha-olefin copolymer is referred to as a linear low density polyethylene; namely, LLDPE.
LLDPEs are employed as such, or as a component in blend with yet other polymers, for the formation of films which are designed for a variety of end use purposes, like films for the consumer market such as household disposable, trash bags and liners; overwrap films and bags for laundry and dry cleaning goods; and shipping and carryout bags for retain merchandising. LLDPE is desirable as a resin for films of such end use designs because of its relatively low cost compared to other resin types such as polyvinylchloride, etc. and because it possesses in combination with this low cost an excellent set of mechanical/physical/chemical properties such as tensile strength, secant modulus, tensile tear strength, puncture resistance, elongation at break, etc.
To this end, LLDPE resins have heretofore been extruded into film layers by both film forming techniques--slot die casting and blown bubble extrusion techniques. However, due to the relatively low melt strength and relatively low dynamic viscosity under low shear rates of an LLDPE resin compared to other polymer types, an LLDPE is more difficult to use as such for fabrication into a film layer by the blown bubble extrusion technique. Hence, when a LLDPE resin is used in a blow bubble extrusion technique for film formation, the processing conditions must be more carefully controlled within a narrower window of operating conditions and certain limitations must be observed upon the dimensions to which the film layer of a LLDPE can be produced, particularly that of its film thickness. Such limitations that must be observed with an LLDPE as utilized in a blown bubble extrusion technique for film formation further limit the rate of film production compared to that at which other types of polymer could be produced to film by a blown bubble extrusion technique.